Older Drivers at a Crossroads

by David A. Morena, W. Scott Wainwright, and Fred Ranck

Design and operational intersection treatments that simplify traffic movement and driving decisions can improve safety for motorists, both young and old.

(Above) The use of overhead lane arrows in Toledo, OH, smoothes traffic flow by helping all motorists, including older drivers, find the proper lane as they approach the intersection.

Research into the needs and capabilities of older drivers consistently reveals a list of visual, physical, and cognitive limitations that develop with the aging of the Nation's population. Weakened vision, reduced reaction times, and limited dexterity, though not predictable for any one individual, are expected for the aging population in general. Couple these vulnerabilities with expanding life expectancy and transportation researchers predict an increasing number of older people with transportation difficulties. In response to the challenges, the transportation community can and is engineering road systems to address these declining abilities and ease the burden on older drivers.

Nowhere are the effects of these demographic changes, and the need for engineering assistance, more evident than at the common meeting of roads—at-grade intersections. According to the FHWA Highway Design Handbook for Older Drivers and Pedestrians (FHWA older driver handbook), "The single greatest concern in accommodating older road users, both drivers and pedestrians, is the ability of these persons to negotiate intersections safely." Maneuvering a vehicle through an intersection requires the visual acuity, physical dexterity, and mental processing capacity to make simultaneous decisions regarding lane choice, vehicle speed and alignment, braking, acceleration, and continuous vehicle positioning relative to other vehicles in the intersection. And the most challenging aspect of intersection negotiation, the FHWA older driver handbook continues, is performing left turns during the permitted (steady circular green) signal phase.

Historically, because most of the driving task is visual, early efforts to improve the road system for older drivers focused on traffic control devices, such as bigger and more reflective signs, brighter markings, and better placement of signs and signals—solutions that address the visual challenges faced by many older drivers. But, there are also design and operational changes to the road system that can actually address some of the cognitive challenges faced by older drivers. Among these cognitive challenges is the need to constantly process and prioritize multiple streams of changing information at once, such as other vehicle movements, pedestrian movements, signs, pavement markings, and lane alignments.

To help motorists navigate intersections safely, transportation agencies can employ a wide range of intersection design and operational techniques, ranging from traffic signal timing and left-turn only signals to installation of roundabouts. All of these techniques, including those discussed in this article, have the potential to reduce crashes and improve safety for the overall driving population as well as older drivers.

A Successful Track Record

FHWA safety analysts recently examined the results of several studies on intersection improvements implemented in Iowa, Michigan, and in France. The analysts posed a simple thesis: Any treatment that simplifies traffic movement and driving decisions for the average driver will provide an even greater benefit to older drivers.

The FHWA safety analysts contacted the authors of the intersection improvement studies listed below and asked them to rerun their data to assess how their study treatments specifically affected older motorists. Although not all of the projects were rigorous studies, the authors were asked to repeat whatever research method they used to calculate crash reductions in general to calculate companion results specific to older drivers.

A study performed by AAA Michigan in 2004 found that the Michigan Department of Transportation improved signal visibility at 33 intersections along Woodward Avenue corridor in Detroit by increasing the size of the traffic signal lens from 20 centimeters to 30 centimeters (8 inches to 12 inches) in diameter and by adding new signals and repositioning existing ones to be more in line with motorists' sight lines. These modifications resulted in a 17-percent reduction in crashes involving drivers between the ages of 25 and 64 —but a 31-percent reduction in crashes involving drivers 65 and older.

According to the same AAA Michigan study, the cities of Detroit and Grand Rapids, MI, substantially reduced crashes at 13 intersections that had been experiencing a high number of left-turn crashes, by installing separate left-turn lanes, along with signal phasing for the turn. As a result, the cities achieved a 66-percent reduction in head-on, left-turn crashes involving drivers younger than 65—but a 73-percent reduction for drivers 65 and older.

The Iowa DOT recently has begun converting roadway corridors, under certain traffic loads, from four-lane to three-lane cross-sections. This change in roadway cross-section provides a safer driving environment by simplifying driving tasks for left-turn movements onto and off the mainline, both at intersections and at driveways. Iowa State University researchers studied 14 of these converted corridors and documented a 24-percent reduction in the crash rate for all drivers—but a 28-percent reduction for drivers 65 and older. Eight corridors in Michigan were modified similarly, with similar results: The improvements yielded a 25.4-percent crash-rate reduction for all drivers—but a 37-percent reduction for drivers 65 and older.

Four-lane to three-lane corridor conversions in Iowa, such as the one shown here, have reduced crashes by 24 percent—and even more for older drivers.

The city of Troy, MI, modified yellow and red clearance intervals on all of its traffic signals to meet current Institute of Transportation Engineers (ITE) guidelines, and then reviewed crash results at the 20 intersections with the highest traffic volumes. The ITE-recommended clearance intervals emphasize an all-red period to protect motorists who enter the intersection near the end of yellow from conflict with cross-traffic. The new signal timing worked: total crashes dropped 18 percent for all drivers—but 42 percent for drivers 65 and older.

Another treatment, the indirect left turn, prohibits motorists from turning left at the intersection proper. Instead drivers are directed to a U-turn, either on the mainline or on a crossroad, eliminating the need to turn in the face of oncoming traffic. In April 1992, a Michigan DOT study documented that, where implemented in Michigan, the indirect left turn yielded an injury-crash reduction of 45 percent for all drivers. The author later reworked his study, using age breakpoints at 20, 40, and 60 years of age. The injury crash reduction for drivers aged 60 and older was 52 percent.

Lastly, the Bretagne-Pays de la Loire district in the French Republic, populated by 6 million people with 25 percent aged 65 or older, has extensive experience with the use of roundabouts. The presence of approximately 4,000 roundabouts offers an opportunity to compare the experience of older drivers in roundabouts versus other intersection types. In analyzing the French district's crash data, safety analysts from FHWA noted that drivers 61 and older were involved in 15.5 percent of the injury crashes at conventional intersections but only 12.4 percent of the injury crashes at roundabouts. The analysts' conclusion: Roundabouts appear to pose less of a safety problem to seniors than other intersections—in that country. One of the benefits to older drivers is that the severity of the crash is less, which is especially significant given their frailty. (See "Roundabouts and Older Drivers" on this page.)

Preparing Motorists In Advance

One strategy to enhance safety at intersections is to prepare motorists well in advance. According to the FHWA older driver handbook, older drivers generally do not react more slowly to events that are expected; however, in dealing with unplanned events, such as finding one's vehicle in the wrong lane at an intersection, the handbook says, older drivers "take significantly longer to make decisions about the appropriate response than younger road users, and this difference becomes more exaggerated in complex situations." Engineers can use any one of several strategies to prepare motorists for safe actions at crossroads.

Roundabouts and Older Drivers

Of the many changes that come with age, the following are particularly relevant for drivers as they negotiate intersections: narrowing of the visual field and the area of visual attention; decreased visual sensitivity to motion; a decline in the abilities to filter out less important information and continuously refocus on what is the most critical information; a decline in the abilities to perform multiple tasks simultaneously and process information from multiple sources; a disproportional increase in perception- reaction time with increase in complexity of the driving situation; and loss of the head, neck, and trunk flexibility needed to rapidly scan an intersection.

In short, older drivers need more time to perceive and evaluate situations, more time to make decisions and take action, and simpler, narrower scenes to take in—all needs that are well served by the lower traffic speeds and less complex vehicle paths of roundabouts. Events play out more slowly at roundabouts, with ample time for all intersection users to anticipate and adjust to the movements of other vehicles and pedestrians. Vehicle/vehicle conflicts and vehicle/pedestrian conflicts are reduced by about three-quarters and two-thirds, respectively, in single-lane roundabouts, and to a somewhat lesser extent in multiple -lane roundabouts.

Typical circulating speeds in Clearwater's four one-lane roundabouts such as this one are 18 to 21 kph (11 to 13 mph), and typical approach speeds (measured at the crosswalks) are 23 to 26 kph (14 to 16 mph).

Roundabouts are less complicated than conventional intersections controlled by stop signs or traffic signals for several reasons: Traffic threats come from only one direction. Color-coded signal bulbs and ever-cycling phases normally are not used in roundabouts. And the approaching driver need only scan straight ahead for pedestrians and about 30 degrees to the left for slow-moving approaching circulating traffic. Above all, roundabouts eliminate the need to judge gaps in fast head-on opposing traffic. Because older persons are more fragile, crash severity is especially significant. Roundabouts, intrinsically by design, limit vehicle speeds on entry and in the circulating lane. Roundabouts also separate opposing vehicle paths, thereby virtually eliminating the most serious types of crashes that occur at conventional intersections: head-on, left-turn, T-bone, and red-light running. Crashes that do occur at roundabouts tend to be either rear-end crashes or else low-angle, low-energy merging crashes with low closing speeds of 8 to 16 kilometers per hour, kph (5 to 10 miles per hour, mph). Because kinetic energy increases exponentially with velocity, roundabout crashes dissipate far less energy than those severe crash types at conventional intersections: right angles with closing speeds of 56 to 89 kph (35 to 55 mph) or head-on crashes with closing speeds of 113 to 177 kph (70 to 110 mph).

According to ongoing studies from FHWA and the National Cooperative Highway Research Program, roundabout injury crashes are reduced about 75 percent and fatalities by 90 percent or more compared with those at conventional intersections.

—Ken Sides, City of Clearwater, FL

Clearwater, FL, has the highest proportion of population aged 65 or older of any U.S. city with populations of 100,000 or more. Clearwater has built five roundabouts and has seven currently in design and five more in the pipeline, for a total of seventeen. All but the first one were proposed by—and strongly supported by —residents.

Advance street name signs can help a driver who is searching for and intends to turn at a specific intersection prepare for the turn by providing advance notice of the upcoming intersection. Advance street name signs provide this critical information ahead of the decision point, reducing the need for visual searching, and freeing the driver to concentrate on both the mechanics of the turn and conflict avoidance with other vehicles.

Stand-alone signs that simply show the street name, with or without an additional legend, often are used in white-on-green format in advance of signalized intersections. Recent practice, now required by the 2003 Manual on Uniform Traffic Control Devices (MUTCD), is to add a distance, or the legend "Next Signal" to the sign. For a nonsignalized intersection, black-on-yellow street name plaques are commonly attached below intersection advance warning signs. Application of this countermeasure has become more common. Many State departments of transportation (DOTs) and local highway agencies have begun to install advance street name signs before both signalized and nonsignalized intersections on their major roadways.

Advance traffic signal warning signs with flashers can alert motorists who are traveling at high speeds on expressways or other high-speed roads and may be caught off guard by traffic signals. The strategy combines a passive sign, such as "Prepare to Stop When Flashing" or a similar message, with an active flasher connected to the traffic signal controller. The flasher starts flashing a few seconds before the change from green to yellow and continues through the red interval and, in some cases, a few seconds into the green signal as the queue dissipates. At least 14 State DOTs use advance warning signs with flashers before traffic signals on high-speed roads, including California, Illinois, Indiana, Iowa, Kentucky, Maryland, Minnesota, Missouri, Nebraska, Nevada, New Jersey, Ohio, Pennsylvania, and Virginia.

The net effect of this early warning on driver behavior appears to be positive. The Minnesota DOT reports that at a pilot site fitted with advance traffic signal warning signs with flashers, red light violations decreased by 29 percent for all traffic and by 63 percent for trucks. At six intersections studied by the Maryland DOT, advance warning flashers have helped reduce right-angle crashes by 62 percent and rear-end crashes by 36 percent. Like advance street signs, traffic signal warning signs provide drivers, including older drivers, with critical information ahead of the decision point and prepare them for a potential stop before the signalized intersection is reached.

Ohio researchers who studied the treatment in their State caution that some drivers tended to use the advance warning to speed up and try to beat the light. But Traffic Engineering Section Chief Scott Thorson with the Nevada DOT cites the benefit of this countermeasure. The Nevada DOT uses advance warning flashers extensively throughout the State, on any roadway with a regulatory speed limit of 89 kilometers (55 miles) per hour or higher. "The onset of the yellow signal indication creates a dilemma for motorists on high-speed approaches to a traffic signal," Thorson says. "The motorists must decide if they have enough time to stop or if they should proceed through the signal. Nevada DOT installed these systems originally to help motorists know that the green phase of the signal is going to end and that they need to prepare to stop because the signal is going to change to red. We have seen a positive change in the behavior of the traffic stream and a reduction in crashes as a benefit of this type of advance warning system, compared to no advance warning. We are believers."

One of the easiest ways to simplify intersection driving is to post the name of the crossroad well in advance, as shown in these two photos.

Advance lane use signs indicate the mandatory or allowable use of some or all lanes when approaching an intersection. Advance lane use signs offer older drivers pertinent information at an earlier point in their approach to an intersection, giving them additional time to move their vehicles to the desired location. Commonly used throughout the country, advance lane use signs are often ground-mounted at the side of the road, but for maximum impact at critical locations, they are placed above the lanes to which they apply. The earlier the information is supplied to the driver, the better, and the best location, as recommended by the MUTCD, is either in advance of the tapers or at the start of added turn lanes. Even where advance placement is not practical and the lane use signs are mounted overhead at the intersection, their bold symbolic designs still can provide valuable assistance to drivers approaching the intersection.

Advance pavement-marking messages can be especially beneficial to road-weary drivers. Drivers who become drowsy can develop tunnel vision, where their ability to drive is reduced to the most basic driving task—following the path of the roadway. In this condition, even though a driver's peripheral vision and sign-reading skills may be restricted, he or she may still respond to messages painted on the pavement. Messages placed on the pavement can vary from "Left Lane Closed Ahead" to "Signal Ahead" to interstate or State route markers. Practically any regulatory, warning, or guide sign message can be supplemented by the same message in a pavement marking to provide an extra measure of communication with the driver.

Advance warning signs with flashers, coordinated with traffic signal operation, are common in at least 14 States. An example from Naperville, IL, is pictured here.

European countries have been using pavement messages extensively for decades, and this concept is coming into more extensive use in the United States. For example, route shield pavement markings such as shown in the photo of Orlando, FL, now are being used in at least five States (Orlando, FL; Maryland; New York State Thruway Authority; Ohio; and Virginia) to give enhanced guidance to motorists.

Providing or upgrading illumination at intersections can be especially important to drivers with declining vision, as a disproportionate percentage of crashes occur at night: nationally, half of all fatal crashes occur at night, even though people travel approximately 70 percent fewer miles at that time than they do during the day. Researchers in Illinois, Iowa, Kentucky, and Minnesota who have studied the effects of intersection lighting in their States, particularly in rural areas, all report similar results—a reduction in nighttime crash rate or nighttime crash frequency of 25 to 50 percent.

"Nothing says 'intersection' to the motorist more than an upcoming group of poles and signs during the day or an oasis of light during the night," says Mike Kamnikar, Minnesota DOT District 2 traffic engineer, who credits illumination as at least partially responsible for reducing crashes at a particular intersection in his own district. Not only is the Minnesota DOT proactively lighting rural intersections on its own roads, says Kamnikar, but "we keep a supply of light poles in our maintenance yard, just so we can respond quickly to local agencies that need the poles to light other rural intersections."

Traffic messages painted on the pavement in the driving lane, in full view of oncoming motorists, can provide an extra measure of communication with the driver. (Left) Motorists are alerted to an upcoming signal in Irvine, CA. (Right) Route markers on the pavement approaches to this intersection in Orlando, FL, help guide motorists to interstate access.

Simplify the Turning Decision

The FHWA older driver handbook lists several reasons why executing a left turn in the presence of oncoming vehicles becomes more difficult for drivers as they age. Foremost among them is the age-related decline in the ability to detect a perceived change in the size of a moving object. It is this visual information—the perceived change in the size of an approaching vehicle—that drivers use to judge the speed and distance of that vehicle, and ultimately, whether the gap in time and space in advance of the approaching vehicle provides enough time to execute the turn. Given this inherent difficulty with the left turn, why not simplify or even eliminate that left-turn task? A small collection of countermeasures do just that.

Restriping the corridor, from four lanes (two in each direction) to three (one in each direction plus a shared left-turn lane) can increase safety in a roadway corridor—often without a loss of travel speed through the corridor. The safety potential of conversion to a three-lane cross-section (also called road diets) was so compelling to Iowa DOT officials, based on studies done in Minnesota, Montana, and Washington, that Iowa DOT made this project type a staple of its agency's older driver program at the program's inception in 1999.

"Four-lane roadways that have no separate left-turn lane for driveways or intersections often have poor safety records, as well as a reputation for inefficient traffic movement" says Tom Welch, Iowa DOT safety engineer. According to Welch, for low to moderate traffic volumes—up to 15,000 vehicles per day—a road with a single high-quality through lane in each direction, unobstructed by left-turn vehicles waiting for gaps, can handle the traffic demand quite well, for most of the hours in a 24-hour day.

The extension of left-turn lane markings into an intersection, shown here in Overland Park, KS, is a common treatment to help guide multiple lanes of turning traffic through intersections.

And generally the three-lane roads are safer. Safety research in Iowa and Michigan, reported earlier in this article, confirms the sizeable crash reductions reported in Minnesota, Montana, and Washington for this reduction in through lanes. A look at the demands on a left-turning driver helps explain why: on the four-lane road, turning left from the mainline requires a motorist to idle in the high-speed, left-through lane of traffic, judging speed and gaps in the oncoming two lanes of traffic. Turning from a side road onto the corridor also is a daunting task, requiring a motorist to judge speed and gap availability in all four lanes of traffic on the main road. When the road is converted to a three-lane cross-section, however, the left-turning tasks become predictably easier, as the turning motorist must judge only one or two traffic streams, rather than two or four as noted above.

In a State ranked 4th in the Nation in percentage of population over age 65, feedback from Iowa's public has been positive. Welch says, "Older road users—drivers and pedestrians as well—report that they are much more comfortable crossing and using these roadways after they have been converted from four lanes to three."

At a conventional signalized intersection, left-turning traffic is given ample time per traffic signal cycle during which to make the turn, but much of this time coincides with through traffic so the driver must navigate through gaps in the oncoming traffic. Generally, a safer strategy, but one that allows fewer vehicles to turn per cycle, is to prohibit left-turn traffic when oncoming through traffic has the green light. In protected only left-turn phasing, the left turners usually are given a much shorter but "protected" period to make their turn, taking their cue from a green arrow. The tradeoff for improved safety is less left-turn throughput and additional left-turn delays. Across the country, agencies take different approaches to this tradeoff. The North Dakota DOT takes a safety-based approach, in which protected only left-turn phasing is installed at nearly all approaches with posted speed limits greater than 64 kilometers per hour (40 miles per hour).

Most signalized roadway corridors experience a significant dropoff in traffic volume during the late night and early morning hours. In reaction to this dropoff, a common strategy for road agencies is to switch at least some traffic signals from the red-yellow-green mode to a flashing mode at night. This strategy usually places the main street signal on flashing yellow, thus not requiring traffic to stop, and the side street traffic on flashing red, although sometimes all-way flashing red is used. Although this approach generally reduces travel delay, motorists react less uniformly to the flashing signal operation. For late-night/early-morning corridors that are operated on flashers and are experiencing left-turn or angle traffic crashes, a return to normal operating traffic signal mode—that is, removing the flashing operation—is likely to reduce total crashes.

In Winston-Salem, NC, a return to normal operation reduced total crashes by 33 percent and right-angle crashes by 78 percent, during the hours of the day that the flashing operation was or had been in effect. In 1984, the Road Commission for Oakland County (Michigan) also documented that removing signals from flash mode improved road safety. The commission then followed up with a countywide policy prohibiting offpeak flashing signal operation at major intersections.

A variety of design strategies actually eliminate the direct left turn at intersections and route left-turning motorists through a median U-turn (Michigan), a right-bearing loop (New Jersey's "jug handle"), or some other method. Even though these strategies require vehicles to travel extra distance, Florida (2001) and Michigan (1992) DOT research both show substantial safety benefits for eliminating the direct left turn at divided highways, although the Florida findings are confined to six- and eight-lane roads.

Installation of street lighting at this rural Minnesota intersection helped to stop a pattern of nighttime fatal and injury crashes.

In a roundabout design, what would have been a left-turn movement becomes a right-turn exit off the circulating road. Five State DOTs—Kansas, Maryland, Minnesota, New York, and Wisconsin—now consider a roundabout alternative in every new design or major redesign of an intersection. Of these States, Kansas DOT has 43 existing roundabouts (8 of which are on the State highway system) and several in the construction and design phases; Maryland DOT has built nearly 60 roundabouts on the State system, with another 30 more in progress; and the New York State DOT has built 18, with 100 more in the design stage (70 of which are on the State system).

Some States without formal policies are active in roundabout construction, such as Colorado and Washington. According to Brian Walsh, State traffic design and operations engineer from Washington DOT, "At one point, I counted no less than 41 jurisdictions that have a bona fide roundabout on the ground and no fewer than 5 agencies that have at least five roundabouts apiece."

This added left-turn lane was part of a city-wide, low-cost safety initiative in Winston-Salem, NC.

Simplify the Turning Mechanics

Engineering strategies also can be used to address some of the other physical limitations that accelerate with age. Range of motion and flexibility in the neck and shoulders typically declines with age, making it more difficult physically to scan the roadway environment. Realigning skewed intersections to a less demanding 90-degree configuration may be helpful. That is why in 2004 the Nebraska Department of Roads instructed its designers to consider realignment to 90 degrees whenever they are designing or retrofitting a major intersection.

Particularly where there is more than one turning lane, or where the turn angle is not 90 degrees, a dotted-line marking to delineate the turning path through the intersection can help guide drivers young and old. Marking extensions assist drivers in maintaining a sufficient distance from other vehicles turning abreast in a multilane turn, avoiding collision with a median nose, and moving into the proper lane upon exiting the intersection. Retroreflective or internally illuminated raised pavement markers are well suited for this application, providing a bright, all-weather navigation aid.

Providing separate left-turn lanes at intersections and major driveways can benefit motorists in two ways. Most important, the two travel movements (straight and turn) travel at different speeds, and separating the two speeds reduces the potential for rear-end crashes. Also, when drivers must judge gaps and turn in the presence of oncoming traffic (permitted signal phasing) exclusive left-turn lanes provide the drivers with a safe haven in which to wait for an acceptable gap. A safe haven is a particular benefit to drivers with declining vision and cognitive skills, who might occupy that lane longer than the average user. According to the older driver handbook, senior drivers preparing to turn left often prefer to wait for larger gaps, and tend to position their vehicles farther back in the intersection such that they actually require larger gaps to make their turn. Studies in Winston-Salem, NC, and Detroit and Grand Rapids, MI, have shown large crash reductions at locations where left-turn lanes were added to the intersection approach.

A full-width approach shoulder in advance of a commercial driveway, as required by a Michigan DOT field office, provides the older driver with the same pressure-free turning movement as is provided by a right-turn lane. Michigan DOT

Although separate left-turn lanes may be desirable, widening the roadway to provide an additional lane may be impractical or prohibitively expensive, or the additional width may adversely affect pedestrian crossings if constructed without a pedestrian refuge. However, State and local DOTs might consider restriping within existing pavement as a potential solution, particularly in urban areas with lower speeds. Left-turn lane widths as narrow as 2.4 meters (8 feet) and through lanes ranging from 2.7- to 3-meters (9- to 10-feet) wide have been found to be workable in some cases—certainly not as good as normal 3.4- to 3.7-meter (11- to 12-foot) lanes, but safer overall than providing no left-turn lane at all.

As documented in the 1990 National Cooperative Highway Research Program (NCHRP) Report 330 Effective Utilization of Street Width on Urban Arterials, using narrower lanes on urban arterials in order to provide left-turn lanes is a very widespread and successful practice. Montgomery County, MD, is one of many jurisdictions that have restriped 14.6- to 15.2-meter (48- to 50-foot) wide urban arterial pavements to provide five lanes, including a center left-turn lane, to improve both capacity and safety.

Providing a separate lane for right-turn traffic also offers advantages that benefit older drivers. Similar to the discussion earlier on left-turn lanes, adding a separate right-turn lane segregates the straight and turning traffic movement and has the potential to reduce rear-end crashes. Left-turning drivers use their turn lane to wait for a gap; however, the right-turn drivers use their separate lane to lower their speed to make a comfortable turn. Making a right turn at slower speeds is particularly helpful to older drivers who are more likely than younger drivers to suffer from physical and muscle degeneration that make turning—or turning quickly—difficult. According to a recent news release by the U.S. National Institutes of Health, "Today, 35 million people—13 percent of the U.S. population—are 65 and older, and more than half of them have radiological evidence of osteoarthritis in at least one joint. By 2030, an estimated 20 percent of Americans—about 70 million people—will have passed their 65th birthday and will be at increased risk for OA [osteoarthritis]." Osteoarthritis and other arthritic and motion-hindering illnesses can inhibit the task of turning and gripping the steering wheel.

As part of its program to design for older drivers, Iowa DOT has added more and longer right-turn lanes in recent years.

Extra pavement for turning traffic can be provided at driveways as well as intersections. The Michigan DOT has a standing policy that developers must supply a short shoulder taper leading to a commercial driveway. The Muskegon Transportation Service Center, a field office of Michigan DOT, pursued a variation on this shoulder widening requirement, requesting that commercial developers provide a full-width shoulder, the length of which varies based on speed limits. A 4.6-meter (15-foot) taper then is included at the end of each shoulder. "Contractors and developers have bought into the requirements because the price of the additional asphalt is offset by reduced labor costs," says Transportation Engineer Tim Terry, in the Michigan DOT Muskegon Transportation Service Center. Once the change was made, according to Terry, "motorists have been using the full shoulders as we had anticipated. [The shoulder tapers] have allowed drivers to make a right turn at a comfortable speed, out of the way of through traffic, which seems to be especially beneficial for elderly drivers."

Whether dealing with a sizeable median in Iowa (top) or a narrow median in Phoenix, AZ (bottom), offsetting the left-turn vehicles will give the drivers a better vision of oncoming traffic and simplify their turning decision.

Improve Sight Distance For Turning Traffic

When left turns are allowed at an intersection, and particularly when those turns are not protected against oncoming traffic, clear sight distance to approaching vehicles is crucial. One design variable that has been shown to improve sight distance for left-turning traffic is to move or offset the turn lane to the left, farther into the median or at least closer to the line of oncoming vehicles. Engineers create a "positive offset" by moving a left-turn lane so far to the left that left-turning drivers can see all the way past the vehicles in the opposing left-turn lane. Positive offset can be particularly helpful for older drivers who, according to the FHWA older driver handbook, tend not to optimize their position in the turn lane, and then have difficulty judging the speed of oncoming vehicles.

Some State DOTs and local agencies offset left-turn lanes on their divided roadways with wide medians. North Dakota DOT, for instance, began offsetting turn lanes wherever possible in 1999. The Wisconsin DOT designs positive offset left-turn lanes on its high-speed, divided intersections, at both nonsignalized and signalized intersections.

Engineers in Phoenix, AZ, have been building offsets into their left-turn lanes for decades, starting with a dozen intersections in the 1970s. The city's commitment to offsetting left turns shows in several ways. "At a few key intersections," says Traffic Engineer Jim Sparks with the city of Phoenix, "we build intersections wide enough for double left turns. However, instead of using the double, we offset drivers nearly one whole car width, which gives them perfect vision."

Phoenix also excels in the more common urban environment, in which tight right-of-way constricts the addition of pavement, and the existing median width tends to be minimal. Even without curb reconstruction, Phoenix has found ways to create at least some sight advantage for left turners at these locations by restriping within existing pavement. In this situation, according to Sparks, "the striping goal is to offset the left-turn drivers sufficiently from their normal positioning so the driver's eye at least lines up with the outside of the opposing left-turn vehicle. We can do this, simply by getting the opposing cars offset approximately 2 feet [0.6 meter] from one another, which means both drivers' eyes have no obstruction from opposing left-turn traffic."

Turning left safely off the main road is one challenge. Turning left safely onto the main road is quite another. Normally engineers and designers try to provide sight lines for turning vehicles by clearing brush, trees, and, to the extent possible, manmade objects that lie between the roads at each quadrant of an intersection. A problem arises, however, where a right-turn lane has been provided adjacent to the mainline through lane. In this case, the right-turn vehicle, especially if it is a large vehicle, may block the line of sight and view of any trailing mainline vehicles for the crossroad left-turn vehicle. Frequent left-turn movements from this crossroad, coupled with heavy truck or bus traffic using the mainline right-turn lane, are a recipe for trouble.

The solution is to offset the right-turn lane anywhere from 1.2 to 2.4 meters (4 to 8 feet) so the driver of the left-turning vehicle on the crossroad can make one final clear sighting down the mainline, past the right-turning vehicle, before moving his or her own vehicle onto the mainline pavement. District 12 of the Kentucky Transportation Cabinet (KYTC) adopted this strategy and now tries to design an offset right-turn lane as needed at all new schools in the district. "I'm sensitive to the possibility of large vehicles blocking small vehicles," says Transportation Engineering Branch Manager Charles Neeley with KYTC District 12. "This strategy directly attacks that problem, providing a valuable line of sight, and often at a relatively low cost."

Offsetting the rightturn lane, as shown in this photo from Michigan, provides crossroad traffic with a better view of oncoming mainline traffic, even when large trucks are in the right-turn lane.

A Win-Win Approach

As illustrated above, engineers and designers have at their disposal a wide array of strategies and techniques they can use to improve the safety of traveling through intersections—for all motorists, and older drivers in particular. According to FHWA Chief Highway Safety Engineer Rudy Umbs, "Intersection safety improvements as a whole, though helpful to all drivers, can be especially beneficial to our aging drivers."

For More Information

Contact personnel and source documents related to the crash reduction statements made in this article for specific intersection-related countermeasures can be found as follows:

David A. Morena is the safety and traffic operations engineer at the FHWA Michigan Division. Morena developed the engineering agenda for the 2004 North American Conference on Elderly Mobility, which included much of the information presented in this article.

W. Scott Wainwright is a highway engineer on the MUTCD team in the FHWA Office of Transportation Operations. Prior to joining FHWA in 2001, he was chief of traffic and parking for Montgomery County, MD, where he worked as a traffic engineer for 27 years after starting his career in that field with the Connecticut DOT. He is a registered professional engineer and a certified professional traffic operations engineer.

Fred Ranck, P.E., P.T.O.E., is a safety engineer for FHWA's Safety and Design National Technical Service Team delivering technology and technical support to FHWA division offices and State DOTs across the country. He also is a member of FHWA's MUTCD team. His experience prior to FHWA includes 12 years as city traffic engineer for Naperville, IL, manager of the Highway Traffic Safety Department of the National Safety Council for 6 years, and county traffic engineer for DuPage County, IL, for 12 years.